This invention relates to an improved device for measuring diametral changes in a cylindrical member. It relates particularly to a device, known as a diametral extensometer, for measuring slight diametral changes in a cylindrical member, such as a valve stem, that can be used for determining the axial loading on the cylindrical member.
In many industries, it is important to measure the variable dynamic axial loads that may be imposed on a cylindrical member or shaft. This is especially true in the nuclear power industry where remotely operable valves are used extensively and monitoring of the various operating parameters of the valves is required by the nuclear power regulating agencies.
It has been observed that one of the best ways to monitor the dynamic forces and events that occur during the operation of a valve is by measurement of the valve stem axial loads using either axial or diametral extensometers.
It is well known that one can calculate the axial load or stress in a valve stem, or any other similar member, by measuring changes in the diameter of the valve stem. The ratio of the diametral change to axial elongation, referred to as Poisson's ratio, is known and available for most materials. Therefore, by measuring the diametral changes in the valve stem using a device such as a diametral extensometer, axial strains and valve stem axial loads can be easily calculated and determined.
One such device described in U.S. Pat. 4,911,004 is designed as a clamp adapted to fit around an exposed portion of a valve stem or cylindrical member to measure diametral strains. Any diametral strains in the valve stem would cause the clamp to deflect. This deflection of the clamp was then measured and calibrated to indicate diametral and axial strains. The measurement of clamp deflection requires that the clamp be flexible enough to produce a reasonable levelof output signals. However, such flexible clamps were often difficult to calibrate in order to produce linear measurements in response to the actual axial loads and strains in the valve stem. Such devices are difficult to set-up, calibrate and remove in a plant environment.
U.S. Pat. No. 5,123,283 describes a device which measures diametral strain directly, thus eliminating the problems associated with a flexible clamp device. Improved accuracy and simplicity of field assembly are, however, still desired for nuclear power plant applications.